Multi-wavelength laser source based on two optical laser beat signal and method

ABSTRACT

In the system of the present invention, two DFB laser outputs are combined in a first stage to produce a beat signal. The two main channels interfere with each other to form beat signals. This combined signal is then used as the seed to create multi-channels through optical fiber non-linearity in a multiplier stage.

This application is a continuation of U.S. patent application Ser. No.10/015,753, filed Dec. 17, 2001, now U.S. Pat. No. 6,826,207, issuedNov. 30, 2004, the entire contents of which is incorporated by referenceherein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to optical communication systems andparticularly to optical laser sources with multiple lasing wavelengths.More particularly still, it provides multi-channel laser signals byutilizing the beat signal of two lasers followed by a non-linear fibermultiplier.

2. Prior Art of the Invention

Dense wavelength division multiplexing (DWDM) offers a very efficientmethod to exploit the available bandwidth in the low attenuation band ofthe optical fiber. In this technology, the enormous available bandwidthis chopped into a number of parallel wavelength channels, where eachchannel carries data up to a maximum rate compatible with electronicinterfaces. Furthermore, different protocols and framing may be used ondifferent channels. This is very similar to frequency divisionmultiplexing (FDM) used for radio and TV transmissions. As technologyprogresses the number of feasible channels in the total band increases.Early WDM systems used only 4 to 16 channels, while new systems aretargeting more than 100 channels.

The low attenuation wavelength band includes different wavelengthsub-bands. The first band used in modern optical communications iscalled Conventional Band or C-Band. This band includes wavelengthchannels from 1520 to 1565 nm. As demand for more bandwidth increased,the number of channels in the C-Band could not provide the capacityrequired by modern telecommunication networks. Therefore, longer andshorter wavelength channels were introduced. Wavelengths covering 1565to 1610 nm form the Long Band or L-Band, while 1475 to 1520 nm from theShort Band or S-Band.

In the transmitter side of a WDM system, there are a number of differentlaser sources with different wavelengths. Each data channel is modulatedon one of the wavelength channels and all the wavelength channels arethen multiplexed and transmitted via the same optical fiber. At thereceiving end, each channel must be demultiplexed from the set ofwavelength channels. An optical receiver, then, will demodulate datafrom each channel. The capacity of a WDM system increases as morewavelength channels are provided. It is therefore desirable to increasethe number of channels, decrease channel spacing and increase the totalwavelength window.

Present DWDM systems need a large number of laser sources as well astechniques to modulate data signals on each source, combine, demultiplexand detect each data stream. The present invention addresses theimportant requirement for laser sources. In particular, it provides amulti-wavelength laser source that simultaneously furnishes a number ofwavelength channels.

Currently, laser sources used in DWDM systems are exclusively of thesingle-wavelength variety. Distributed Feed-Back (DFB) lasers,Fabry-Perot lasers and ring lasers are some of the main technologies.Each wavelength supported in the system has a dedicated laser and itsancillary electronics. In the last few years and still today, themajority of lasers used are capable of emitting light only at a fixedwavelength. Increasingly, however, designs are making use of tunablewavelength lasers, which have broader spectral range and can operate atany point within that range. The primary drawback of both of thesedevices, however, is the sheer number that is required to satisfy highchannel count systems proposed for the future optical network. At thesame time, it is important to be able to lock the center wavelength ofeach laser source to a specific wavelength. This is mainly due to thefact that if there is any drift in the wavelength of a laser, it caninterfere with the adjacent wavelength channel. This fact imposes apractical limitation on the number of discrete laser sources that may beplaced in a very tightly spaced wavelength channel system providing alarge number of channels. As a result, a multi-wavelength laser sourcethat can provide an efficient and simple wavelength locking, system ishighly desirable.

SUMMARY OF THE INVENTION

This invention provides a novel design which provides simultaneously anumber of wavelength channels. The present design requires only a singlewavelength locking mechanism to tune and lock the entire set of channelsto the ITU standard grid. Furthermore, the present system is able toprovide wavelength channels in all three S, C and L bands.

In the system of the present invention, two DFB laser outputs arecombined in a first stage to produce a beat signal. The two mainchannels interfere with each other to form beat signals. This combinedsignal is then used as the seed to create multi-channels through opticalfiber non-linearity in a multiplier stage. The multiplier stage expandsthe two initial channels to cover the target wavelength band. A ComblikeDispersion Profile Fiber (CDPF) system is used in the multipliersection. The channel spacing of the resulting channel set follows thechannel spacing of the two initial DFB lasers.

Accordingly, a multi-wavelength laser source (MWLS) system, comprisingfirst and second monochromatic lasers having first (f₁) and second (f₂)lasing frequencies, respectively, means for amplifying combined signalsof said first and second lasers and means for multiplexing the amplifiedcombined signals to yield Comblike multi-channel laser signals separatedfrom each other by a frequency equal to the difference between f₁ andf₁.

The system as defined above, said means for multiplying comprising aplurality of serially interconnected optical fiber sections each havingpredetermined propagation characteristics for said amplified combinedsignals, said predetermined propagation characteristics beingpropagation mode, dispersion and length.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred exemplary embodiments of the present invention will now bedescribed in detail in conjunction with the annexed drawing, in which:

FIG. 1 shows a block diagram of a Multi-Wavelength Laser Source (MWLS)according to the present invention;

FIG. 2 illustrates dispersion versus length of a Comblike DispersionProfile Fiber System used as the Multiplier in FIG. 1 for the C-BandMWLS with 100 GHz spacing;

FIG. 3 shows the simulation result for the C-Band MWLS with 100 GHzspacing; and

FIG. 4 shows the actual experimental result for the C-Band MWLS with 100GHz spacing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a Multi-Wavelength Laser Source (MWLS) systembased on two DEB laser beat signal is shown. The system starts with twosingle channel DFB laser sources 10 and 11 and multiplies the number ofchannels to cover a target wavelength band such as C, L, S or acombination of thereof. The ultimate channel spacing between adjacentchannels is dictated by the spacing of the two original lasers 10 and11. Consequently, a very good locking technique on the original lasersinsures wavelength looking in the whole set of output channels. Tuningof the whole set of channels to the ITU grid is also based on the timingof the two starting lasers. This means that the driver circuits 12 and13 for the original seed lasers 10 and 11 need to tune andwavelength-lock the two lasers to the ITU grid. As a result, this MWLSdesign simplifies the wavelength toning and locking which otherwisewould have had to be performed for each individual laser. Thus, in thecase of a few hundred channels, it is easy to see the benefits of thecentral tuning and locking of only two lasers.

The output signals of the DFB lasers 10 and 11 are combined in a firststage coupler 16 after passing through polarization controllers (PCs) 14and 15. The PCs are used at the output of the lasers to assist in theefficient interference of the two lasers. A high power optical amplifier17 then amplifies the combined signal at the output of the combiner 16.This amplification enhances the non-linear effects of the subsequentoptical medium, (the multiplier 18), since non-linearity of the opticalmedium (such as an optical fiber) is proportional to the power of thesignal.

The interference of the two laser outputs forms a “beat” signal.Consider two monochromatic lasers, the first 10 with central frequencyf₁ and optical intensity I₂, and the second 11 with central frequency f₂and optical intensity I₂, their complex wave-function at some point inspace isU ₁(t)=I ₁ ^(1/2)exp(j2πf ₁ t),andU ₂(t)=I ₂ ^(1/2)exp(j2πf ₂ t),respectively.

The interference wave-function then is the sum of the two, which isU(t)=U ₁(t)+U ₂(t)=I ₁ ^(1/2)exp(j2πf ₁ t)+I ₂ ^(1/2)exp(j2πf ₂ t),

Therefore, the intensity of the combined signal, I(t), would beI(t)=I ₁ +I ₂+2(I ₁ I ₂)^(1/2) cos [2π(f ₂ −f ₁)t].

This shows that the intensity varies sinusoidally at the differencefrequency |f2−f₁|, which is called the “beat frequency.” This signal isa good candidate for use alongside fiber non-linearity effects toprovide a wide coverage through channel multiplication of the initiallaser sources. This is mainly due to of the fact that this methodprovides very short optical pulses.

The novel design further relies on the multiplier stage 18, whichexpands the coverage of the wavelength channels by “multiplying” theoriginal two channels using non-linear effects in optical fibers. Themultiplier 18 preferably consists of a series interconnection of opticalfibers with different chromatic dispersion characteristics, which iscalled a Comblike Dispersion Profile Fiber (CDPF) system. The multiplierdesign is first done through analytical calculations as well assimulations. An efficient multiplier is one that can provide a widecoverage with enough laser signal-to-noise ratio or equivalently goodextinction ratio. If the system is properly designed, longitudinal modescan be preserved in the wide band signal, thus enabling continuous wavechannels to be realized. In fact, the multiplier creates very shortoptical pulses from the beat signal and at the same time expands thewavelength coverage. Going through optical fiber non-linear effects suchas Cross Phase Modulation (XPM), Self Phase Modulation (SPM) and FourWave Mixing (FWM), high power short optical pulses can generate a wideband coherent signal, which is also called a “Super Continuum” (SC).

The generalized nonlinear Schrödinger equation is used to describe thepropagation of the optical pulse in an optical fiber:

$\frac{\partial{E\left( {z,t} \right)}}{\partial z} = {\left\lbrack {\hat{D} + \hat{N}} \right\rbrack \cdot {{E\left( {z,t} \right)}.}}$Where E(z,t) denotes the electrical field of the light wave. Thenon-linearity is shown by the {circumflex over (N)} operator, whichdepends on the nonlinear index and represents photon elastic andinelastic scattering processes, such as Rayleigh and Raman scattering inthe fiber. {circumflex over (D)} is the dispersion operator whichrelates to the dispersion parameter of the fiber. This equation includesnonlinear processes such as SPM, XPM, FWM, Raman effects, the first andsecond order of group-velocity dispersion (GVD) and fiber attenuation.

In the present exemplary embodiment an MWLS for 100 GHz spaced system inthe C-Band, i.e. 40 Channels, is considered. In this system, the two DFBlasers 10 and 11 are tuned to wavelength channels on the ITU grid around1550 nm. As shown to FIG. 1, the outputs of the lasers are directedthrough the polarization controllers 14 and 15 to enhance laser beatquality when they interfere. The 3-dB coupler 16 is then used to combinethe output of these two single channel lasers. The combined outputsignal is then amplified to about 800 mW range by the high power ErbiumDoped Fiber Amplifier (EDFA) 17. This high power signal goes through theCDPF system 18 to be expanded. To design the CDPF stage 18, we need tosolve the Shrödinger equations of the optical fiber system to insureproper expansion as well as preservation of the longitudinal modes.

FIG. 2 shows the CDPF system 18 designed for this example, whichconsists of five stages of Dispersion Shifted Fiber (DSF) and SingleMode Fiber (SMF) with different chromatic dispersion characteristics. Asshown for this example, L₁=1.1 km, L₂=1.1 km, L₃=20 m, L₄=1 km, and L₅=1km, where the associated dispersion values are D1=−0.399 ps/km/nm,D2=0.402 ps/km/nm, D3=16 ps/km/nm, D4=0.402 ps/km/nm, and D5=−0.399ps/km/=m; all at 0.1550 nm. In this CDPF system 18 the first, second,fourth and fifth segments are DSF and the third segment is SMF.

The high power beat signal at the output of the amplifier 17 and theCDPF 18 shown in FIG. 2 are necessary to realize the MWLS but not alwayssufficient. In order to expand the channel coverage, we need to suppressthe Stimulated Brillouin Scattering (SBS) in the system. SBS reflectsand scatters some of the injected power. This reduces the effectivepower launched to trigger non-linear effects. SBS frequency depends onthe germanium concentration in the optical fibers. In the CDPF structure18, since the concentration of germanium is different in each segment,it can suppress the SBS growth through the system. However, since theSBS threshold is lower for narrow band signals, it is important toreduce the SBS in the system. An improvement in the present exemplaryembodiment is achieved by modulating the DFB lasers 10 and 11 by a verylow frequency signal (around 30 kHz), which does not affect systemoperation. Experimental results, as well as simulations, showsignificant reduction in SBS.

Finally, the simulation result is shown in FIG. 3, while the MWLS outputfrom the experimental results in the laboratory is shown in FIG. 4. Asshown, for this instance of the MWLS the C-Band is covered withContinuous Wave (CW) channels spaced at 100 GHz. In this example, eachoutput channel power is around 12 mW.

1. A multi-wavelength laser source (MWLS) system, comprising: (a) firstand second monochromatic lasers having first (f₁) and second (f₂) lasingfrequencies, respectively; (b) an amplifier for amplifying combinedsignals of the first and second lasers; (c) a multiplier the formultiplying in number the amplified combined signals to yield comblikemulti-channel WDM laser signals separated from each other by a frequencyequal to the difference between f₁ and f₂; (d) first and second driversadapted to tune the first and second lasers so as to tune the comblikemulti-channel WDM laser signals.
 2. The system as defined in claim 1,said means for multiplier comprises a plurality of seriallyinterconnected optical fiber sections, each section having respectivepredetermined propagation characteristics for the amplified combinedsignals.
 3. The system as defined in claim 2, wherein the predeterminedpropagation characteristics are propagation mode, dispersion and length.4. The system as defined in claim 3, wherein the plurality of seriallyinterconnected fiber sections comprises five serially interconnectedfiber sections having lengths L₁, L₂, L₃, L₄ and L₅, respectively, L₁being a first section, and L₅ being a last section.
 5. The system asdefined in claim 4, wherein a third fiber section of the five seriallyinterconnected fiber sections is a single mode fiber (SMF) section. 6.The system as defined in claim 5, wherein the first, second, fourth andfifth fiber section of the five serially interconnected fiber sectionsare dispersion shifted fiber (DSF) sections.
 7. The system as describedin claim 6, wherein L₁=1.1 km, L₂=1.1 km, L₃=20 m, L₄=1 km and L₅=1 km.8. The system as defined in claim 7, wherein the first, second, third,fourth and fifth fiber section have associated dispersion valuesD₁=−0.399 ps/km/nm, D₂=0.402 ps/km/nm, D₃=16 ps/km/nm, D₄=0.402ps/km/nm, and D₅=−0.399 ps/km/nm; respectively.
 9. The system asdescribed in claim 8, wherein f₁ and f₂ correspond to wavelengths in thevicinity of 1550 nm.
 10. The system as defined in claim 2, wherein theserially interconnected optical fiber sections have differentconcentrations of Germanium.
 11. The system as defined in claim 10,wherein the first and second drivers are adapted to modulate the firstand second monochromatic lasers, respectively, using very lowfrequencies.
 12. The system as defined in claim 1, wherein the first andsecond drivers are adapted to wavelength-lock the first and secondmonochromatic lasers.
 13. The system as defined in claim 1, wherein thecomblike multi-channel WDM laser signals cover at least one of C-band,S-band, and U-Band.
 14. The system as defined in claim 1 comprisingfirst and second polarization controllers coupled to the first andsecond monochromatic lasers, respectively, and a coupler coupled to thefirst and second polarization controllers and to the amplifier forcombining signals from the first and second monochromatic lasers. 15.The system as defined in claim 1, wherein the amplifier is adapted toprovide a level of amplification sufficiently high to enhance non-lineareffects during the multiplying the amplified combined signals.
 16. Amethod comprising: producing a first monochromatic signal having a firstlasing frequency f₁ and a second monochromatic signal having a secondlasing frequency f₂ using first and second lasers, respectively;combining the first monochromatic signal and the second monochromaticsignal to produce a combined signal; amplifying the combined signal;multiplying in number the amplified combined signal to yield comblikemulti-channel WDM laser signals separated from each other by a frequencyequal to the difference between f₁and f₂; tuning the comblikemulti-channel WDM laser signals using first and second drivers, thefirst and second driven being drivers of the first and second lasers,respectively.
 17. A method according to claim 16 comprisingwavelength-locking the first and second lasers using the first andsecond drivers.
 18. A method according to claim 16 comprisingpropagating the first monochromatic signal and the second monochromaticsignal through first and second polarization controllers, respectively.19. A method according to claim 16, wherein the multiplying comprisespropagating the amplified combined signal through a plurality ofserially interconnected optical fiber sections, each section havingpredetermined propagation characteristics for the amplified combinedsignal, the combined signal being amplified to a level of amplificationsufficiently high to enhance non-linear effects during the multiplyingof the amplified combined signal.
 20. A method according to claim 16,wherein the serially interconnected optical fiber sections havedifferent concentrations of Germanium.
 21. The method as defined inclaim 20 further comprising modulating the first and secondmonochromatic lasers with the first and second drivers, respectively,using very low frequencies.
 22. A multi-wavelength laser source (MWLS)system, comprising: (a) first and second monochromatic lasers havingfirst (f₁) and second (f₂) lasing frequencies respectively and producingsignals having first and second optical intensities respectively; (b) anamplifier for amplifying combined signals of the first and secondlasers; (c) a multiplier for multiplying in number using non-lineareffects the amplified combined signals to expand the coverage of thewavelength channels so as to yield comblike multi-channel WDM lasersignals comprising a plurality of more than two channels separated fromeach other by a frequency equal to the difference between f₁ and f₂.